The present invention relates to high energy gas lasers, and more particularly, to a large, high pressure, gas laser power amplifier.
The requirements of large energies on the order of tens of kilojoules, and short pulse durations of a nanosecond or less create unique design problems in high power laser amplifiers. With the large energies, it is desirable to use high flux densities to minimize size and cost. At present, two kinds of lasers are widely used where extremely high output energies are required: solid neodymium glass lasers with a 1.06 .mu.m wavelength and carbon dioxide lasers with a 10.6 .mu.m wavelength. The neodymiun glass lasers have been popular because of a well established, fifteen-year-old technology. However, solid state lasers such as neodymium glass, are limited in capability because of their low efficiency of approximately 0.17%, and a low pulse repetition rate capability of approximately 6 times per hour. The more recently developed high pressure electron beam controlled CO.sub.2 laser, is unique in being relatively efficient, on the order of 3% to 5%, together with providing a short pulse of 0.5 to 1 ns duration, a high repetition rate, and high power output. It also has the valuable features of rapid waste heat removal and employment of an easily replaced, damage free gain medium.
The efficient creation of a population inversion in CO.sub.2 -nitrogen gas mixtures which lead to the 10 .mu.m radiation depends on two circumstances. First, the vibrational level of nitrogen is readily pumped by inelastic electron molecule collisions. Second, this vibrational level is nearly coincident with the 001 level of CO.sub.2 and the excitation of the N.sub.2 is shared quickly with the CO.sub.2. The electrical excitation efficiency for the 001 level depends upon the specific gas mixture employed, but under optimum conditions is approximately 50% for short pulse operation. The transitions of interest are the 9 and 10 .mu.m transitions to the 020 and 100 levels. Up to 50% of the inversion is theoretically available from the excitation. However, the rate at which the energy can be extracted is limited by the rate of gas kinetic collisions between the rotational sublevels of the upper and lower vibrational levels which are the total level populations. An overall efficiency of 30% had been demonstrated for continuous operation of CO.sub.2 lasers. However, for sub-nanosecond pulse extraction an overall efficiency of 2% is more typical.
It has been known that very short pulse amplification in CO.sub.2, along with compact power amplifiers, could be achieved by operation at or above atmospheric pressure, but there remains a problem of discharge stability at these higher pressures. This problem was solved by the TEA, that is, transversely electrically excited, atmospheric pressure CO.sub.2 lasers, utilizing electrode discharges. However, the electric field to pressure ratio required to maintain the ionization in the discharge was far too large for most efficient vibrational excitation of nitrogen. Electron beam control provided the solution to this problem, making the CO.sub.2 laser an efficient short pulse amplifier. An externally generated electron beam enters the amplifier medium and provides the required ionization independently of the discharge voltage. The applied voltage, therefore, can be chosen to effect the most efficient discharge pumping for the gas mixture and total gas pressure applied. The discharge thus formed has the additional advantage of being stable against arcing.
It is apparent, therefore, that the CO.sub.2 gas laser offers many advantages for the production of high energy pulses. Electrical efficiency is high. Short pulse extraction efficiencies of greater than 10% are possible by the use of multiple pulse energy extraction. The gas medium of the CO.sub.2 laser essentially eliminates concerns of damage to the medium that can arise in solid state lasers such as neodymium glass, and the gas is easily exchanged by flow to provide cooling. Thus the laser medium is not the controlling element for high repetition rates. Long pulse CO.sub.2 lasers have been operated at repetition rates of 750 Hz. Short pulse CO.sub.2 lasers can be operated at repetition rates ranging from a few Hz up to 50 Hz.
CO.sub.2 laser systems of a given power output are more compact than those of neodymium glass. In high output CO.sub.2 lasers it is desirable to use high flux densities to minimize size and cost. The population inversion in CO.sub.2 lasers occurs between vibrational levels in the electronic ground state. Superimposed on the vibrational levels are the rotational levels and, on a nanosecond time scale, the energy exchange rate between rotational levels can substantially affect the amount of energy that can be extracted, as well as the temporal shape of the amplified pulse. In addition to gain saturation effects, energy extraction and pulse shape in the nonosecond regime also required due consideration of bandwidth and coherent phenomena. For efficient performance, high gas pressure is employed, since the rotational energy transfer time varies inversely with pressure, and line broadening is directly proportional to pressure. Simultaneous extraction of energy on several lines is also very desirable, but must be weighed against the increased cost complexities it entails.
High gas pressure is also desirable in order to obtain a high density of energy storage in relatively small size amplifiers. Energy storage density for a given gas mixture and temperature is proportional to the product of pressure and small signal gain. Since pumping efficiency and parasitic oscillation place limits on small signal gain, high energy storage density generally requires high pressure operation. However, the pressure is limited by the necessity of higher discharge voltages and gas optical breakdown, the breakdown threshold varying inversely with pressure.
As noted above, two types of high pressure CO.sub.2 amplifiers are currently in general use: those employing self-sustained electrical discharges, and those which use an external source of high energy electrons to provide the gas ionization for maintaining the electrical discharge. At their present state of development, each type has special virtues and limitations. Self-sustained discharge amplifiers are limited in size to a gap width times pressure product of about 20 cm atmospheres, must operate at a higher than desirable electric-field-to-number-density ratio, and utilize gas mixtures not optimal for nanosecond pulse amplifiers. They can, however, produce a uniform amplifier medium and are generally simpler than E-beam sustained amplifiers, which require the addition of a high voltage electron gun. E-beam sustained amplifiers can be made to operate with much larger PD values. They can be operated with both optimal gas mixtures and optimal E/N values.
Uniformity in the amplifier medium is difficult to achieve in very large amplifiers. Any nonuniformity in spatial distribution of ionization from the external electron beam results in nonuniformities of the discharge. Magnetic fields from the discharge current can severely affect the trajectories of the electrons from the external gun and thus cause nonuniform ionization. Because of the size limitations on self-sustained types, the present invention employs an electron beam.
Gas mixtures chosen for nanosecond pulse amplifiers differ from those used in most applications. Nitrogen is used in laser gas mixtures because it readily exchanges its vibrational energy with the upper laser level in CO.sub.2 molecules and in effect, provides a very selective channel for funneling energy from the electric discharge into the desired state of the CO.sub.2 molecule. The CO.sub.2 upper state level population essentially comes into equilibrium with the nitrogen levels during the pumping time of a few microseconds. During the nanosecond energy extraction time, however, no significant energy transfer from the nitrogen occurs, and the energy stored in the nitrogen molecules is wasted. For this reason, the optimal mixtures for nanosecond pulse amplifiers use less nitrogen.